| Marine equipments surve in a low temperature environment and suffer from the co-corrosion of seawater and atmosphere in a long time.Because of the harsh service conditions,steels for marine engineering equipments must possess such comprehensive properties as high strength,high toughness and excellent low-temperature impact toughness and resistance to the co-corrosion of seawater and atmosphere,and the harsh conditions of marine engineering equipments have put forward higher requirements for the quality of welding process and welding joint.Currently,the researches about microalloying in welding process which will improve microstructure,mechanical properties or second-phase particle of weld metal mainly focused on the electrode coating.The method of flux-cored wire welding has such advantages as higher production efficiency,good welding quality and better adaptability of welding position,whose droplet transfer form is different from traditional welding methods,such as arc welding.Most of welding process of the flux-cored welding are multi-pass welding with high heat input,resulted in different microstructure and properties of the welded joint.Besides,the experimental research on the modification of 600 MPa grade HSLA weld metal is also rare.This subject based on a certain alloy system of flux-cored wire,added within mixed RE or Cu elements,prepared a serie of flux-cored wires for 600 MPa grade HSLA steel,then performed welding experiments to obtain the effect of RE and Cu on the properties of weld metal,respectively.In this paper,600 MPa grade flux-cored wire with different addition of RE or Cu was made by the method of cold rolling steel strip forming.Microstructure,mechanical properties and electrochemical properties of the weld metal were analyzed by means of metallographic microscope,SEM,EDS,XRD,electrochemical accelerated corrosion and mechanical properties test,respectively.According to the experimental results,the influence mechanism were summarized,which would provide reference for the microalloying of the flux-cored wire.The experimental results shows that the addition of RE elements has obvious influence on the microstructure and mechanical properties of weld metal.The acicular ferrite increases gradually with the increasing amount of RE addition,while the uniformity of tissue and grain size decreased within excessive RE addition.Within the appropriate content,the low-temperature impact toughness,hardness,yield strength and tensile strength were improved significantly,Re reached 606 MPa while Rm reached 753 MPa,and showing a trend of rising first and then decreasing.The second-phase particles with appropriate size also have a certain increase,which is beneficial to the grain nucleation.The addition of Cu can promote the formation of acicular ferrite to some extent,but it can also lead to the formation of the phenomenon of grain boundary enrichment,which has no obvious effect on the yield strength and tensile strength as well as the low-temperature impact toughness of the weld metal,yet the mechanical properties generally develop in an unfavorable direction.In terms of electrochemical characteristics,the addition of the RE and Cu have a certain improved effect on the corrosion resistance of the weld metal,electrochemical test results show that the more RE is added,the lower corrosion currents and the larger radius of capacitive reactance curves are,indicated higher corrosion resistance.Within an optimal range of Cu addition,roughly 0.28%~0.72%,the weld metal have a lower corrosion current and higher impedance,with stable corrosion resistance.Therefore,in the process of microalloying for flux-cored wire,RE played an important role in the improvement of overall performance of the weld metal,Cu also has a certain role in promoting the corrosion resistance,and slightly improve the structural properties of the weld metal has not been significantly improved by it. |
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